Occasionally LLDB runs into contexts where the

target is stopped in a C++ or Objective-C method
but the "self" pointer's valid range actually
doesn't cover the current location.  Before, that
was confusing Clang to the point where it crashed;
now, we sanity-check and fall back to pretending
we're in a C function if "self" or "this" isn't
available.

llvm-svn: 143676